1. Field of the Invention
The present invention relates to a chemical mechanical polishing apparatus used to transfer a wafer and to polish the surface of the wafer. The present invention relates to, in particular, a chemical mechanical polishing apparatus capable of enhancing workability in wafer transfer and uniformity in wafer polishing.
2. Description of the Related Art
Conventionally, various types of chemical mechanical polishing (to be referred to as `CMP` hereinafter) apparatuses have been used for polishing a wafer surface. There has been proposed, for instance, a CMP apparatus capable of enhancing polishing accuracy by applying pressure to a wafer by a pressure fluid and pressing the wafer against a turning table with a uniform manner (in Japanese Patent Application Publication No. 7-67665).
FIG. 1A is a cross-sectional view showing a part of a conventional CMP apparatus and FIG. 1B is a plan view showing a wafer holding surface of the apparatus in FIG. 1A. FIG. 1A shows a condition in which a wafer is held on a wafer holder. As shown in FIGS. 1A and 1B, a back pad 23 is attached onto the back of a stage 22. The stage 22 and back pad 23 are provided with first gas holes 22a extending from the back pad 23 side in the direction perpendicular to the surface of the back pad 23 and with a cavity 22b connected to the first gas holes 22a. There is also provided a connection section 26 including the second gas hole 22c for suctioning a gas within the cavity 22b and/or supplying a gas into the cavity 22b on the upper wall portion of the stage 22. The connection section 26 is connected to a pump (not shown). As shown in FIG. 1B, the first gas holes 22a are arranged on a circle around the center of the back pad 23 and stage 22 and having about half the radius of that of the back pad 23 and stage 22.
Further, a holding ring 24 is disposed on the end faces of the stage 22 and back pad 23 to surround them. The holding ring 24 slightly protrudes downward from the surface of the back pad 23 and is designed to hold the peripheral edge surface of the wafer 25. A wafer holder 21 for the CMP apparatus is thereby constituted.
In wafer polishing, first, the wafer 25 is held within the holding ring 24 of the wafer holder 21 and then a vacuum pump connected to the connection section 26 is driven. Thus, the cavity 22b and the gas holes 22a are evacuated and the wafer 25 is vacuum-drawn on the back pad 23 which becomes a wafer holding surface. Next, while the wafer 25 is attached onto the back pad 23, the wafer 25 is transferred onto a turning table (not shown). A pressure pump connected to the connection section 26 is then driven, thereby pressing the wafer 25 against the surface of the turning table and subjecting the surface of the wafer 25 to CMP processing. Thereafter, the vacuum pump connected to the connection section 26 is driven again to thereby suction the wafer 25 onto the surface of the vacuum pad 23 and transfer the wafer 25 to a predetermined position.
As stated above, according to the conventional CMP apparatus, the suction and transfer of wafer 25 and the CMP processing for pressing the wafer 25 against the table are carried out on the same line using the gas holes 22a and the like provided in the wafer holder 21. The conventional apparatus is, thereby, designed to enhance workability in the transfer of the wafer 25 and performance in CMP processing, i.e., polishing uniformity.
However, in case of transferring and CMP processing the wafer 25 using the CMP apparatus including the above-stated wafer holder 21, the same gas holes 22a are used to suction and press the wafer 25. Due to this, appropriate suctioning conditions during transfer as well as appropriate pressure conditions for CMP processing are disadvantageously limited. The reasons are described as follows.
As shown in FIGS. 2A and 2B, it is assumed that gas holes 22d are formed close to the center of the stage compared with the gas holes 22a shown in FIGS. 1A and 1B. When the wafer 25 is pressed against the table 27, the center of the pressed wafer 25 is curved downward (concave-shaped) as shown in FIG. 3B. Thus, when CMP processing is conducted, polishing agent tends to enter between the table 27 and the wafer 25, with the result that the entire wafer 25 including the central portion thereof can be uniformly polished.
Meanwhile, if the wafer 25 is suctioned on the back pad 23 to transfer the wafer 25 once again after CMP processing, the center of the suctioned wafer 25 is curved upward (convex-shaped) as shown in FIG. 3A. At this moment, the surface of the wafer 25 has been smoothed as a result of the CMP processing and adhesion between the table 27 and the wafer 25 increases. Due to this, the suction force between the wafer holding surface and the wafer 25 is insufficiently low and the wafer 25 is often detached from the wafer holder 21. Transfer errors may occur, accordingly.
Further, as shown in FIGS. 4A and 4B, it is assumed that gas holes 22e are formed close to the peripheral edge side of the stage compared with the gas holes 22a shown in FIGS. 1A and 1B. If the wafer 25 is suctioned on the back pad 23, the peripheral edge portion of the suctioned wafer 25 is curved upward (concave-shaped) as shown in FIG. 5A. Due to this, the peripheral edge portion of the wafer 25 is suctioned after CMP processing, thereby making it possible to reduce adhesion between the table 27 and the wafer 25 and to prevent the occurrence of transfer errors.
In the meantime, if the wafer 25 is pressed against the table 27, the peripheral edge portion of the pressed wafer 25 is curved downward (convex-shaped) as shown in FIG. 5B. This makes it difficult for a polishing agent to enter between the table 27 and the wafer 25 during CMP processing. As a result, the surface of the wafer 25 is not polished uniformly.
As can be seen from the above, preferable conditions for suctioning the wafer 25 onto the wafer holder 21 differ from those for pressing the wafer 25 against the table 27. Normally, therefore, the gas holes 22a are arranged on a circle around the center of the back pad 23 and stage 22 and having about half the radius of that of the back pad 23 and stage 22 as shown in FIGS. 1A and 1B. By doing so, it is intended to overcome or reduce the problems shown in FIGS. 2A, 2B, 3A and 3B as well as those shown in FIGS. 4A, 4B, 5A and 5B. As a result, appropriate suctioning condition during transfer and appropriate pressure conditions during CMP processing are disadvantageously narrowed. Hence, workability in transfer and polishing uniformity in CMP processing cannot enhance further.